1. Field of the Invention
This invention relates to a fixing device and an image forming apparatus incorporating the fixing device.
2. Description of the Related Art
As a fixing device used in various image forming apparatuses, such as a copier, a printer, a facsimile machine, a multifunctional machine, etc., that which employs a thin-walled fixing belt consisting of a metal substrate and an elastic rubber layer or the like is known. The low heat capacity thin-walled fixing belt greatly reduces energy consumed for heating thereof, and a warm-up time (e.g., reload time) can be shortened. A time to first print (i.e., a time period from receiving a printing request to ejecting a sheet via preparing and conducting printing based thereon) can also be shortened.
As this type of a conventional fixing device, Japanese Patent Application Publication No. 2007-334205 (JP-2007-334205-A) discloses a device that includes an endless belt (e.g., an fixing belt) 100, a pipe-shaped metal heat conductor 200 disposed inside a loop of the endless belt 100, a heat source 300 disposed in the metal heat conductor 200, and a pressing roller 400 contacting the metal heat conductor 200 via the endless belt 100 forming a nip N thereon as shown in FIG. 15. The endless belt 100 is driven and rotated when the pressing roller 400 rotates. The metal heat conductor 200 guides and allows the endless belt 100 to travel at this moment. Since the endless belt 100 is heated by the heater 300 via the metal heat conductor 200, the entire endless belt 100 can be heated. Thus, the time to first print from a heat standby state can be reduced and moreover the problem of insufficient heat during high speed rotation can be resolved.
Further, Japanese Patent Application Publication No. 2007-233011 (JP-2007-233011-A) also proposes a fixing device that directly (i.e., not via the metal heat conductor) heats the endless belt to further enhance energy efficiency and the time to fast print as shown in FIG. 16.
Further, the pipe-shaped metal heat conductor is sometimes omitted from inside the loop of the endless belt 100, and instead, a nip formation member 500 having a sheet plate shape is provided facing the pressing roller 400. Consequently, since the endless belt 100 can be partially directly heated by the heat source 300 other than a nip formation section of the nip formation member 500, heat transfer efficiency can be greatly improved, thereby reducing power consumption. Because of this, the time to first print from a heating standby state can be further reduced, achieving cost reduction due to omission of the metal heat conductor.
Due to a malfunction or the like (e.g., a system error) of the image forming apparatus, there is a risk that a heat source of the above-described fixing device overheats damaging the fixing belt. To prevent this, a thermostat with a bimetallic strip or the like is provided in the fixing device as a safety device. However, when the temperature of the fixing belt exceeds a prescribed level, power to the heat source is shut off by the thermostat, and a control unit senses a malfunction and stops rotation of the fixing belt, shutting the machine shut down.
However, as shown in FIG. 16, since the heat capacity of the fixing belt is small, the heated fixing belt heats up quickly when heated directly. At the same time, however, the bimetal thermostat has low responsiveness. Consequently, a time lag develops from the time when the fixing belt is overheated to the time when power to the heat source is actually shut off, by which time the fixing belt is likely to have been damaged already.
To prevent this possibility, the thermostat of the safety device can be replaced with a temperature sensor, such as a thermistor, etc. However, such a sensor detects even slight variations in the temperature of the fixing belt, possibly causing the control unit to mistakenly interpret transitory fixing belt temperature overshoots as system malfunctions and shut down the machine even though the machine is operating normally. Frequent shutdowns can be a nuisance. In such a situation, the thermostat can be cooled by a cooling device, such as a fan, etc., but such a configuration increases the cost and the size of the apparatus.